1. Field of the Invention
The present invention relates to a diversity receiving apparatus suitable for a communication system such as a high speed wireless LAN (Local Area Network).
2. Description of the Related Art
In recent years, LAN has been widely used in offices so that information processing apparatuses such as computers that are connected with cables are effectively used. More recently, technologies of wireless LAN have been developed and are becoming common. Although the transmission rate of information of wireless LAN is at most on the order of several Mbps. However, to transmit multimedia data such as moving pictures on the real time basis, a high speed data transmission as high as several ten Mbps has been intensively studied so that it can be used in near future.
One of the major bottlenecks to be considered in increasing the transmission rate of information and widening the frequency band thereof is an influence of a transmission distortion on a multiplexed wave transmission path. In other words, in an environment of which a multi-path wave takes place, it is clear that a fading deteriorates the communication quality of the received signal. Particularly, in a high speed data transmission environment, the fading results in a frequency fluctuation and an inter-code interference as critical problems as well as the influence of the transmission distortion. As the data transmission rate rises, the transmission distortion becomes a dominant noise factor against the inner noise of the receiving system. Thus, even if the transmission power is increased, the transmission quality may not be improved. To solve such a problem, in the high speed wireless data transmission system, some countermeasures against transmission distortion and so forth are taken.
For example, as one of the countermeasures against the transmission distortion and so forth in the high speed data transmission environment, OFDM (Orthogonal Frequency Division Multiplexing) method has been proposed. In the OFDM method, a signal is divided into several sub-carriers and transmitted in parallel on the frequency axis. This system is referred to as multi-carrier communication system. In particular, the carrier intervals can be minimized with an orthogonal function system so that a desired band width is decreased as small as a single carrier.
To convert the time axis into the frequency axis or vice versa in the OFDM method, FFT (Fast Fourier Transform) is used. Thus, it is not necessary to use a plurality of modulating/demodulating devices corresponding to individual sub-carriers. In addition, since the time axis is converted into the frequency axis or vice versa using FFT, the receiving portion that converts a received high frequency signal into an intermediate frequency signal can handle all carriers as one signal wave.
When the OFDM method is used, since the period of a symbol is expanded several times as large as the period of a sub-carrier, the influence of the inter-code interference can be remarkably reduced. However, as long as all carriers are handled as one signal wave, the influence against the frequency selective fading cannot be prevented.
FIG. 1A shows an influence of a frequency selective fading in the case of a single carrier. FIG. 1B shows an influence of a frequency selective fading in the case of a multi-carrier of which the signal shown in FIG. 1A is processed corresponding to the OFDM method. In FIGS. 1A and 1B, the horizontal axis and the vertical axis represent a frequency and a signal level, respectively.
As shown in FIG. 1B, the correlation in the frequency band of each divided carrier unit is high. However, as with the case shown in FIG. 1A, in the entire frequency band, the correlation of levels of individual levels is low. Thus, the influence of the frequency selective fading cannot be prevented. Thus, the influence of the frequency selective fading is a critical problem of the OFDM method.
To solve the above-described problem of the OFDM method, a sub-band division space diversity method has been proposed. In the sub-band division space diversity method, one frequency band is divided into several sub-bands. A diversity combining process is performed for each of the sub-bands so as to remove a dip due to a fading on the frequency axis or the time axis.
Next, with reference to FIGS. 2A, 2B, 2C, and 2D, a process for dividing one frequency band into five sub-bands, selecting higher output level portions from the divided sub-bands, and combining the selected portions is described. In this example, the process is performed by three antenna and receiving portions.
FIG. 2A shows characteristics of sub-bands a1 to a5 formed by the first antenna and receiving portion as the first channel. FIG. 2B shows characteristics of sub-bands b1 to b5 formed by the second antenna and receiving portion as the second channel. FIG. 2C shows characteristics of sub-bands c1 to c5 formed by the third antenna and receiving portion as the third channel.
When the diversity combining process is performed for individual output signals having such characteristics, a portion having a high output level portion is selected from each sub-band of each channel. Thus, as shown in FIG. 2D, an output signal of which the sub-bands c1 and c3 formed by the third antenna and receiving portion, the sub-bands b2 and b4 formed by the second antenna and receiving portion, and the sub-band a5 formed by the first antenna and receiving system is obtained free of dip portions.
Generally, in the high speed data transmission environment, since the amount of information transmitted per unit time is large, there is a tendency whereby a relatively large transmission power is required. For example, to accomplish the communication quality and communication area for the low speed data transmission environment, the transmission power should be increased in proportion to the amount of information.
However, in reality, since the power consumption of the apparatus and the power amplifier module thereof have their limitations, the communication area is deteriorated. To solve this problem, with a plurality of discrete antenna and receiving portions, a fading margin is improved.
For example, when the number of discrete antenna and receiving portions is increased from two to four, assuming that the instantaneous power failure ratio is 0.1%, the required fading margin decreases from 15 dB to 7 dB. This effect is equivalent to the case of which the transmission power is increased by 8 dB. When the number of antenna and receiving portions is increased, characteristic benefits are obtained. However, since the increase of the number of antenna and receiving portions causes the circuit scale and the cost thereof to increase. Thus, when a plurality of antenna and receiving portions are used, it seems that selective combining method that is the simplest method may be properly used.
FIG. 3 is a block diagram showing the structure of a diversity receiving apparatus corresponding to OFDM method using the conventional sub-band division space diversity that has been described.
In FIG. 3, an antenna group 101 is composed of n antennas 1011, 1012, . . . , and 101n that are spaced each other. The antennas 1011, 1012, . . . , and 101n are for example non-directivity antennas such as whip antennas and helical antennas.
The antennas 1011, 1012, . . . , and 101n receive a signal from a remote terminal. The received signal is a signal of which data has been primary modulated corresponding to QPSK (Quadrature Phase Shift keying) method or QAM (Quadrature Amplitude Modulation) and then secondary modulated corresponding to OFDM method.
Output signals of the antennas 1011, 1012, . . . , and 101n are supplied to receiving portions 1021, 1022, . . . , and 102n, respectively. Each of the receiving portions 1021, 1022, . . . , and 102n has a high frequency amplifying circuit, a frequency converting circuit, an AGC (Automatics Gain Control) circuit, and so forth.
The receiving portions 1021, 1022, . . . , and 102n amplify received signals of the antennas 1011, 1012, . . . , and 101n, respectively. Thereafter, the receiving portions 1021, 1022, . . . , and 102n limit their frequency bands, convert the received signals into intermediate frequency signals, and perform an AGC operation for the intermediate frequency signals. With control signals of the AGC circuits of the receiving portions 1021, 1022, . . . , and 102n, the strengths of the received signals of the receiving portions 1021, 1022, . . . , and 102n are detected. The detected strengths of the received signals are supplied to multiplying devices 1071, 1072, . . . , and 107n, respectively.
Output signals of the receiving portions 1021, 1022, . . . , and 102n are supplied to multiplying devices 1031, 1032, . . . , and 103n, respectively. Window signals are supplied from receiving window circuits 1041, 1042, . . . , and 104n to the multiplying devices 1031, 1032, . . . , and 103n, respectively. The multiplying devices 1031, 1032, . . . , and 103n limit an effective symbol period and extract predetermined portions.
Output signals of the multiplying devices 1031, 1032, . . . , and 103n are supplied to FFT circuits 1051, 1052, . . . , and 105n, respectively. The FFT circuits 1051, 1052, . . . , and 105n convert respective parallel signals on the frequency axis into respective serial signals on the time axis and perform an OFDM demodulating process for the serial signals. Output signals of the FFT circuits 1051, 1052, . . . , and 105n are supplied to a branch selecting circuit 108.
Output signals of the FFT circuits 1051, 1052, . . . , and 105n are supplied to received power detecting devices 1061, 1062, . . . , and 106n, respectively. The received power detecting devices 1061, 1062, . . . , and 106n detect the strengths of individual sub-bands of individual branches. Output signals of the received power detecting devices 1061, 1062, . . . , and 106n are supplied to the multiplying devices 1071, 1072, . . . , and 107n, respectively. Output signals of the multiplying devices 1071, 1072, . . . , and 107n are supplied to the branch selecting circuit 108.
The branch selecting circuit 108 compares the strengths of the individual sub-bands of the individual branches and selects a signal of a branch having the highest level. In other words, the AGC circuits of the receiving portions 1021, 1022, . . . , and 102n detect the strengths of signals of the entire frequency bands of the individual branches. The received power detecting devices 1061, 1062, . . . , and 106n detect the strengths of received signals of the individual sub-bands of the individual branches. Corresponding to the obtained strengths, the branch selecting circuit 108 selects a branch.
An output signal of the branch selecting circuit 108 is supplied to a demodulating circuit 109. The demodulating circuit performs a demodulating process for data that has been primarily modulated corresponding to QPSK or QAM method. An output signal of the demodulating circuit 109 is obtained through an output terminal 110.
As another detecting method for strength information of each sub-band, it may be detected with a filter having a very high Q. However, since the Q of the filter is limited, this method is not practical. As another detecting method, the strength information may be detected on the time division basis in a time period shorter than the fading period. However, since the period of an OFDM symbol is long and it takes a time to perform the FFT calculation, this method is not practical. Thus, the above-mentioned method is used to detect strength information.
However, in the conventional diversity receiving apparatus of which the selective combining method is applied for a receiving system corresponding to OFDM method using the above-described sub-band division space diversity, the strengths of received signals of individual sub-bands of individual branches are detected. Corresponding to the detected result, a branch is selected. Thus, circuits that detect the strengths of signals of individual sub-bands should be disposed for individual branches. In other words, in the above-described example, it is necessary to disposed the FFT circuits 1051, 1052, . . . , and 105n and the received power detecting devices 1061, 1062, . . . , and 106n for the individual branches. Thus, according to the related art reference, the size and cost of the apparatus cannot be reduced.
Therefore, an object of the present invention is to provide a diversity receiving apparatus that prevents an OFDM type receiving system from being affected by a frequency fading and that allows the circuit scale to be reduced.
To solve the above-described problem, the present invention is a diversity receiving apparatus for receiving data that has been primarily modulated and then secondarily modulated, comprising a plurality of uni-directivity antennas arranged so that they orient in different directions relative to each other, a signal information detecting means for detecting signal information of all frequency bands of received signals of each of the uni-directivity antennas, a branch selecting means for selecting one of received signals of the individual branches received by the uni-directivity antennas, a secondarily modulated data demodulating means for demodulating data that has been secondarily modulated as the received signal selected by the branch selecting means, and a primarily modulated data demodulating means for primarily demodulating data that has been primarily modulated as an output signal of the secondarily modulated data demodulating means, wherein the branch selecting means is controlled corresponding to an output signal of the signal information detecting means.
According to the present invention, a plurality of uni-directivity antennas are disposed so that they are oriented in different directions. With the uni-directivity antennas, a received wave is separated. Thus, the fading dip decreases. Consequently, it is not necessary to dispose received signal strength detecting means for individual sub-bands unlike with the related art reference. Thus, the circuit scale and cost can be reduced.